The invention relates to electronic devices which use, e.g. alkaline batteries, for the power supply must not be discharged at high currents if the batteries have reached their end of life voltage. Otherwise, the battery may leak and destroy the electronic device before being replaced. Electronic devices having a microcontroller are usually set into a reset mode if the supply voltage drops below a certain threshold. Typically, this threshold value is defined by the minimum supply voltage of the microcontroller. In the reset mode, the microcontroller is kept in a defined state which allows a quick start up if the supply voltage rises again.
FIG. 1a to 1c illustrate a typical behavior of an electronic device which is coupled to a varying power supply voltage VCC. FIG. 1a shows the time-dependent supply voltage VCC, FIG. 1b shows the time-dependent Brownout signal BOR and FIG. 1c shows the current IVCC which is taken by the electronic device at the respective points in time. The electronic device is activated if the supply voltage VCC exceeds the voltage level VOFF. Accordingly, the Brownout signal BOR is set from LOW to HIGH. The electronic device starts operation and continues until the supply voltage VCC drops below the minimum supply voltage threshold VON. Subsequently, the Brownout signal BOR will be set from HIGH to LOW and the electronic device enters the reset state.
There is a small hysteresis between the voltage levels VOFF and VON in order to prevent the electronic device from undesired fast switching between the reset state and the active state. However, the voltage levels VOFF and VON derive from a single threshold voltage which separates the reset state from the active state. If the supply voltage is above this threshold, the electronic device is activated. If the supply voltage is below this threshold, the electronic device is in the reset state. The exact value of this threshold varies very little depending on whether a raising or sinking supply value exceeds the threshold and causes the electronic device to enter the active mode and reset mode, respectively. This is due to the small hysteresis between the voltage levels VOFF and VON. However, within the context of this specification, notwithstanding the fact that there is a small difference between the voltage levels VOFF and VON, they will be regarded as a single threshold because they derive from one and the same threshold value.
The power consumption IVCC of the electronic device (see FIG. 1c) varies if the electronic device is in the active state and drops to zero (according to the IVCC-graph FIG. 1c) if the supply voltage VCC drops below VON. The state of the electronic device is indicated by the Brownout signal BOR. A HIGH Brownout signal indicates the active mode and a LOW Brownout signal indicates the reset state. At a first sight and according to the IVCC-graph in FIG. 1c, the power consumption drops to zero in the reset state. However, the microcontroller's power consumption in the reset state is not equal to zero, it will be a few μA or even higher. This is mainly due to the fact, the microcontroller is kept in a state allowing a fast start up if the supply voltage VCC rises again. The power consumption in the reset state may be too high to prevent the alkaline batteries, which are driving the electronic device, from leaking.
A further risk for battery leakage in a prior art battery-driven electronic device derives from its undefined behavior taking place at low supply voltages. The electronic device starts operation if the supply voltage VCC exceeds VSTART (see FIG. 1a). A Brownout module, which may be a part of the electronic device, starts operating and sets the Brownout signal BOR to LOW, because the supply voltage VCC does not exceed the threshold VOFF. However, at a voltage level VTH which is far below VSTART, there is an undefined operation of the electronic device. The Brownout module provides an undefined Brownout signal BOR. For instance, the Brownout signal BOR rises together with the supply voltage VCC even though the supply voltage VCC is well below VOFF (which is the voltage level for the electronic device to change to the active mode). The undefined Brownout signal BOR is indicated by the dashed line in FIG. 1b. The undefined Brownout signal BOR may lead to an uncontrolled behavior of the electronic device. This can result in reasonable power consumption peaks IPEAK which increase the danger of battery leakage.
There is a danger that the batteries may leak if these are discharged beyond a certain threshold voltage. FIG. 2 illustrates the battery voltage VBAT of a typical alkaline battery during operation of an electronic device. By way of an example only, the electronic device is operated for 3 to 6 months. During this time, the battery voltage VBAT drops from the initial value of 1.5 V to 0.9 V. At a minimum supply voltage of VON=0.9 V (see FIG. 1a), the electronic device enters the reset mode. However, even in the reset mode, there is reasonable power consumption and due to this, the battery voltage VBAT drops further, i.e. from 0.9 V to 0.6 V. This will take approximately 1 to 2 months, according to the example in FIG. 2. For battery voltages below 0.6 V, there is a danger of battery leakage. This situation can last for several years as it is indicated by the tail portion of the battery voltage VBAT curve. Accordingly, the battery may leak and destroy the electronic device before the battery is replaced by the user.